1. Field of the Invention
This invention relates to the field of phased array antenna beam steering and in particular to optical antenna beam steering.
2. Background Art
Phased array antennas, which are used in radar and communications systems, require phase shifters at each radiating element or sub-array of radiating elements to electronically steer the antenna beam. Many types of phase shifters have been developed for phased array applications. The two main classes of phase shifters are the semiconductor phase shifter and the ferrite phase shifter. While both classes of phase shifters exhibit excellent performance when properly designed, the recent success in implementing semiconductor phase shifters in monolithic microwave integrated circuits offers the promise of significantly reducing the cost of large phased array systems. The most common type of monolithic microwave integrated circuit phase shifter is a digital phase shifter which uses the standard bit designs such as the switched line, loaded line, reflection type, and high pass/low pass sections. Many successful digital phase shifter designs have been implemented in the prior art in monolithic microwave integrated circuit form for a wide variety of operating frequencies.
Most monolithic digital phase shifters require a command in the form of an n-bit parallel binary word where n is the number of phase shifter bits. This mandates a minimum of n separate control lines for each phase shifter. Moreover, certain types of digital phase shifters require complementary control lines for each bit, thereby doubling the number of control lines required. Therefore, for large phased array systems, which may include up to ten thousand radiating elements, it is desirable to devise methods which can significantly reduce the number of control lines and/or the amount of information that must be routed to the phase shifters.
A prior art technique used to reduce the number of control lines that must be sent to each phase shifter involves the transmission of serial phase shifter data and subsequent demultiplexing and serial-to-parallel data conversion to provide the appropriate command to a number of phase shifters. A hybrid gallium arsenide optical controller recently developed by National Aeronautics and Space Administration includes a high speed digital fiber optic link, a PIN photodetector, and a MESFET demultiplexer that can distribute serial data to as many as sixteen phase shifters. This approach may be utilized in a wide variety of systems due to its compatibility with existing digital phase shifter technology. However, in this prior art system the circuitry is fairly complex and serial data rates are still high for large phased array systems. Therefore it is desirable to develop an alternate technique which uses the same philosophy of compatibility with existing phase shifter technology while requiring less complex circuitry. This alternate technique should also be compatible with the field of optical signal processing, in order to reduce beamsteering computations.